In general, such a detecting system is so arranged that a detection signal level from the sensor increases in proportion to a rise of the value of the environmental phenomenon such as a smoke density. By this reason, the sensitivity of the sensor is fixed.
With this arrangement, if the sensitivity of the analog sensor is set so as to be optimum for the detection for a range of relatively low smoke density, the detection signal level from the analog sensor will be raised largely even if the smoke density increases only a bit. Therefore, it often occurs that the detection signal is soon saturated, exceeding a dynamic range or full span of devices at later stages such as an A/D converter to which the detection signal is supplied and that the smoke density detection over a change range to be detected can not be assured.
On the other hand, if the sensitivity of the analog sensor is set so as to be optimum for the detection for a range of relatively high smoke density, a change in the detection signal level relative to a change in the smoke density will be small. Therefore, when the smoke density is relatively small, the detection is liable to be influenced by external noises. Moreover, accurate detection can not be assured with an A/D converter for general use having an ordinary resolution.
To solve these problems, it has been proposed as a compromise to set the sensitivity of the sensor so as to be optimum for a range of intermediate smoke density, a bit sacrificing the detection accuracy at the low or high smoke density. An alternative solution comprises a plurality of analog sensors of different sensitivities which are provided in each of detectors, or an A/D converter of high resolution. Such a solution, however, increases a cost of the entire system.
The present invention has been made to overcome the problems as described above, and it is an object of the present invention to provide a detecting system and a detector, in which the sensitivity of an analog sensor is not fixed with reference to a change in an environmental phenomenon (for example, a density of smoke or gas), but it can be changed according to a change in the environmental phenomenon. More specifically, it is an object of the present invention to provide a detecting system and a detector, in which the sensitivity of the analog sensor is set so that the detection signal from the analog sensor may vary within a range not exceeding the dynamic range of devices at later stages such as an A/D converter, whereby the detection accuracy can be improved, even when a single sensor is used, over a wide range of changes in the environmental phenomenon, by effectively using the dynamic range of the devices at the later stages.
The present invention to attain the object features a detecting system a detecting system including an analog sensor provided in a detector for sensing a change in a quantity of an environmental phenomenon and a receiver adapted to receive and process a signal corresponding to a detection level output from said analog sensor for detecting the change in the environmental phenomenon, the improvement comprising a sensitivity setting means which changes the detection sensitivity of said analog sensor between a plurality of sensitivity levels according to the quantity of said phenomenon.
The present invention further features a detector which includes an analog sensor for sensing a change in a quantity of an environmental phenomenon and processes a signal corresponding to a detection level output from said analog sensor to detect a change in the environmental phenomenon, the improvement comprising a sensitivity setting means which changes the detecting sensitivity of said analog sensor between a plurality of sensitivity levels according to the quantity of the phenomenon.